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1
In a small bowl, blend the Roquefort with the butter.
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2
Scrape the butter onto plastic wrap and roll into a 4-inch cylinder.
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3
Refrigerate until firm, about 1 hour.
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4
Slice the butter into 4 disks.
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5
Gently shape the ground sirloin into 4 thick patties.
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6
Using your thumb, make a depression in the center of each patty and fill with a disk of Roquefort butter.
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7
Fold the meat over to encase the butter completely and gently pat the burger into a plump, round patty.
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8
Light a grill.
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9
Thread each onion slice onto a skewer.
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10
Brush the onion and tomato slices with 1 tablespoon of olive oil and season with salt and pepper.
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11
Lightly brush the grate with vegetable oil.
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12
Grill the onions over a medium-hot fire until charred and tender, about 5 minutes per side.
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13
Grill the tomato slices until charred, about 1 minute per side.
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14
Season the burgers with salt and pepper and grill over a medium-hot fire until browned and cooked through, about 6 minutes per side.
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15
Transfer to a platter and let stand for 2 minutes.
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16
Brush the cut sides of the rolls with the remaining 1 tablespoon of olive oil and grill until lightly browned, about 10 seconds.
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17
Set the burgers on the rolls, top with the onion, tomato and arugula and serve.
